Another, hypothetical example of ad-hoc definitions (maybe this is wrong, I’m not an hydraulics engineer): Suppose we are trying to design lubricants for a bearing. We might try to quantify the “slippyness” of a liquid by pouring it on a sloped table of standard length and timing how long it takes to flow off the table. What we really want is viscosity, which (for a Newtonian fluid) is invariant to shear rate and lots of other things. But “slippyness” is not a good abstraction, because it depends on viscosity, cohesion and adhesion forces, etc.
We can tell slippyness is not uniquely constrained because when we change parameters like pour rate, table slope, length, and composition, there is no simple law that relates these different slippyness measures. Force of a Newtonian fluid in rheometer is linear in shear rate.
Later, we could observe that viscosity applies to liquids and gases, whereas slippyness only applies to liquids, giving us a little more evidence that viscosity is a better abstraction. When designing the bearing, matching the viscosity of the lubricant to the bearing forces proves better than using slippyness.
Another, hypothetical example of ad-hoc definitions (maybe this is wrong, I’m not an hydraulics engineer): Suppose we are trying to design lubricants for a bearing. We might try to quantify the “slippyness” of a liquid by pouring it on a sloped table of standard length and timing how long it takes to flow off the table. What we really want is viscosity, which (for a Newtonian fluid) is invariant to shear rate and lots of other things. But “slippyness” is not a good abstraction, because it depends on viscosity, cohesion and adhesion forces, etc.
We can tell slippyness is not uniquely constrained because when we change parameters like pour rate, table slope, length, and composition, there is no simple law that relates these different slippyness measures. Force of a Newtonian fluid in rheometer is linear in shear rate.
Later, we could observe that viscosity applies to liquids and gases, whereas slippyness only applies to liquids, giving us a little more evidence that viscosity is a better abstraction. When designing the bearing, matching the viscosity of the lubricant to the bearing forces proves better than using slippyness.
Yup, you’ve got the right idea. Good example.